Well if all you want is basic SPI/NAT firewall, the Linksys firmware is fine.

If you want to do fancy features like adding iptables rules above what the Linksys GUI allows, then you want other firmware.

I for example have a 2nd IP setup on my WAN interface, and some custom DNAT and SNAT rules so that one particular client seems to be available on this alternate IP, and responds from it. Fancy Linux stuff that you can do fairly easily with some lines of iptables.

Also, not sure if current Linksys firmware has QoS? Haven't used it myself in a while. The custom firmware do have QoS.

I found Hyperwrt_GSv4_Thibor13d and Hyperwrt_GS_Thibor13d
What is the best for my WRT54GSv4?

I read that Hyperwrt_GS_Thibor13d is better.
"I agree- I think the 13d basic version will work fine. I went from Tofu to Thibor in my G V4 this week, and attempting the V4 version got me "timeout error: upgrade failed", but loading up the plain version got me an immediate and easy upgrade."

they both have their own merits, it depends what you want from them, dd-wrt has far more features integrated than mine. the ONLY difference between the normal and v4 version is the code header for flashing from stock v4 firmware and tftp, that's all